1. Field of the Invention
The present invention relates to a high-frequency heating apparatus such as a microwave oven, and particularly to a driving mechanism such as a turntable on which a material to be heated is placed and turned.
2. Description of the Prior Art
In a high-frequency heating apparatus such as a microwave oven, a material to be heated is usually turned so that the entire material is irradiated with high-frequency radio waves as evenly as possible. The material is placed on a turntable provided in a heating chamber, and the turntable is rotated by a driving force from the output spindle of a driving motor provided outside the heating chamber. If the output spindle, which is typically made of a metal, is disposed through the bottom surface of the heating chamber, it acts as an antenna and induces the radio waves to leak out of the heating chamber, posing fire and health hazards outside the heating chamber.
To prevent such leakage of radio waves, as shown in FIG. 16, a choke 3 is formed around the output spindle 2 of the driving motor 1 so that it is placed around a through hole 6 formed in the bottom surface 5 of the heating chamber 4. This choke 3 prevents the radio waves from leaking out of the heating chamber 4. In the figure, numeral 7 represents the turntable, numeral 8 represents a driving shaft for transmitting the rotation of the output spindle 2 to the turntable 7, numeral 9 represents a roller assembly consisting of a roller and a roller support, and numeral 10 represents a cabinet.
On the other hand, Japanese Published Patent Application No. H1-42566 proposes a microwave oven in which the output spindle is made of a material having a low dielectric constant such as a resin or ceramic so that it will not induce the radio waves to leak out of the heating chamber. This microwave oven is thus free from leakage of radio waves and therefore highly safe to use despite having no choke.
These conventional high-frequency heating apparatuses, however, have the following disadvantages:
First, even if the output spindle is made of a material having a low dielectric constant, it tends to attract radio waves, so that a strong electric field is present around the through hole formed in the bottom surface of the heating chamber. Thus, there is a possibility that the radio waves leak out of the heating chamber through the through hole, with the output spindle acting as a medium. In particular, the higher the dielectric constant of the material of the output spindle, the more the radio waves leak. By contrast, the lower the dielectric constant of the material, the less the radio waves leak. This, however, increases the cost of the output spindle.
Second, providing a choke on the bottom surface of the heating chamber for the prevention of leakage of radio waves not only increases the number of necessary components, but also increases the production cost by requiring an extra step of fixing the choke on the heating chamber by welding. Moreover, the choke requires extra space below the heating chamber, and thus makes it difficult to make the cabinet as compact as possible.
Third, fluid such as juicy contents of food or water dripping down through the through hole formed in the bottom surface of the heating chamber, when brought into direct contact with the output spindle of the driving motor or other electric components, may cause irregular rotation of the driving motor or imperfect insulation.
Fourth, the turntable burdens the output spindle with a load. This increases the load to be borne by the driving motor, and thus causes irregular rotation, or necessitates reinforcement of the output spindle. In addition, as the temperature inside the heating chamber rises, the heat is conducted to the driving motor and shortens its useful life.